Myasthenia gravis and neuromuscular autoimmune diseases

Principal investigators
Prof. Jan J.G.M. Verschuuren (Neurology), Prof.dr. Silvère M. van der Maarel, Jaap J. Plomp (Neurology), Maartje G. Huijbers (Neurology)

The myasthenia gravis (MG) research group is part of the expert centre for Neuromuscular diseases in the LUMC. Our research group consists of basic and clinical researchers from the department of Neurology and the department of Human Genetics.  
The expert centre for Neuromuscular diseases at the LUMC is one of the Health Care Providers (HCP) of the EURO-NMD European Reference Network.

MG is a neuromuscular autoimmune disease hallmarked by fluctuating fatigable muscle weakness. The muscle weakness is a direct consequence of autoantibodies binding to, and interfering with, the function of key neuromuscular junction proteins. The majority (~85%) of patients have IgG1 and IgG3 autoantibodies against acetylcholine receptors (AChR). These antibodies cause disease by crosslinking and internalization of AChR, activation of complement and recruitment of immune cells that results in synaptic damage and impaired neuromuscular transmission. A second group of patients (~5%) have antibodies against muscle-specific kinase (MuSK). MuSK is essential in the formation and maintenance of neuromuscular synapses as it propagates a trophic signal to induce AChR clustering and supports presynaptic motor nerve terminal differentiation. Over the past years we confirmed that MuSK autoantibodies are predominantly of the IgG4 subclass, can induce MG in mice independent from complement or immune cells and that disease severity correlates with IgG4 antibodies against the Ig-like 1 domain of MuSK (Klooster & Plomp et al. 2012 Brain, Huijbers et al 2016 J neuroimmunology). In this preclinical animal model we furthermore confirmed the therapeutic potential of Efgartigimod (a new FcRn inhibitor) for MuSK MG patients (Huijbers & Plomp et al. 2019 Exp Neurol).

While studying the pathomechanism by which the MuSK autoantibodies cause disease we confirmed that the pathogenicity of MuSK MG autoantibodies is directly related to the functional characteristics of IgG4. IgG4 is limited in its ability to activate complement an other immune system components. IgG4 is furthermore functionally monovalent as it has the unique ability to exchange half-antibodies (a process called Fab-arm exchange). We showed that MuSK autoantibodies obstruct MuSK-LDL receptor-related protein 4 (Lrp4) interaction, which inhibits a trophic signalling cascade, impairs AChR clustering, culminating in impaired neuromuscular transmission and muscle weakness (Huijbers & Zhang et al 2013 PNAS). We furthermore isolated monoclonal antibody sequences from patients and investigated their functional characteristics (Huijbers et al. 2019 Neurol, Neuroimmunol & Neuroinflamm). The valency of monoclonal MuSK antibodies dictates their pathogenic effect. In other words, monovalent MuSK antibodies (like Fab-arm exchanged serum IgG4) inhibit MuSK and AChR clustering, whereas bivalent MuSK antibodies (recombinantly produced) stimulate MuSK and partially induce AChR clustering.
Interestingly, a range of other autoimmune diseases has been identified that are associated with predominating IgG4 autoantibodies (Huijbers et al. 2018 Ann N Y Acad Sci).
These diseases share many of the disease mechanism characteristics observed in MuSK MG which suggests them to form a niche among the antibody-mediated autoimmune diseases. Why these diseases are hallmarked by IgG4 autoantibodies is not known. 


Figure 1. Examples of overlap between IgG4-mediated autoimmune diseases pathomechanisms  (Huijbers et al. 2015 European Journal of Neurology).

Small subsets of patients with antibodies against Lrp4 and agrin have also been identified, however still ~5-10% of patients are suspected to have MG based only on their clinical characteristics without any of the currently known autoantibodies. These so-called “seronegative” patients remain subject of analyses for the identification of new antigens. Lastly, autoantibodies against presynaptic voltage-gated calcium channels (VGCC) suffer from a myasthenic syndrome called Lambert-Eaton myasthenic syndrome (LEMS). Approximately, 50% of patients have an underlying small cell lung carcinoma, which is considered the cause of this paraneoplastic syndrome.

Although each of these subtypes of myasthenia are characterized by fluctuating and fatigable muscle weakness several important differences can be observed between the clinical presentations of these diseases. These differences include among others variegated treatment response and muscle group involvement, but are mechanistically poorly understood.

Our research focuses on the following topics:
  1. Characterizing natural course and disease progression in patients with myasthenia gravis. One of the main topics is to characterize disease progression and monitor treatment effects in patients taking part in clinical trials. See https://www.lumc.nl/org/neurologie/research/myasthenie-register/. We provide experimental research support that varies from developing new diagnostic tests to standardized testing of neuro-immunological outcome parameters (Strijbos et al. 2017 Vaccine).
  2. Development and validation of new diagnostic tools for ocular myasthenia gravis. The diagnosis of ocular autoimmune myasthenia gravis can be extremely challenging, as more than half of the patients are seronegative and often other signs of MG are lacking. So far, the function of eye muscle cannot be evaluated directly by objective tools, as they are not accessible for tests like single-fiber EMG or EMG with repetitive stimulation. We are investigating the diagnostic value of imaging techniques, using our 7 Tesla MRI scanner, and the use of new electrophysiological tests, like evoked potentials, to study the characteristics of the eye muscles in patients with suspected MG.
  3. Understanding NMJ homeostasis and preclinical studies in models of myasthenia gravis. Basic knowledge on neuromuscular transmission and synaptic homeostasis is not only valuable for understanding normal physiology, but can also shed light on important aspects of development and progression of myasthenic muscle weakness. It may furthermore identify druggable targets that can strengthen neuromuscular synapses. We therefore use our passive transfer models to study neuromuscular synapses on biological and physiological level.
  4. Unravelling the immunological characteristics of (IgG4) antibody-mediated autoimmunity. IgG4-mediated autoimmune diseases have only recently been recognized. Why these diseases are predominated by IgG4 autoantibodies is not known. Why these patients develop autoimmunity is also poorly understood. Our studies are aimed to answer these questions.
  5. Identify new antigens in seronegative myasthenia gravis. Approximately 5-10% of MG patients have clinical characteristics of MG but serological analysis cannot identify the currently well-known autoantibodies. To facilitate diagnosis in these patients and improve therapeutic strategies we aim to identify new antigenic targets. Identification of new antigenic targets might furthermore yield insight on new key players at the neuromuscular synapse.

Future perspectives
Our ambition is, together with our broad network of collaborators, to advance knowledge on healthy and diseased neuromuscular junction physiology, understand the aetiology of MG-related autoimmunity and develop new therapeutics for MG. We welcome all students and scientist interested in our work to contact us for any questions regarding positions or collaborations.

Relevant websites
https://www.spierziektencentrum.nl/
https://mgexpertisecentrum.nl/
https://www.lumc.nl/org/neurologie/research/myasthenie-register/
https://www.lumc.nl/org/neurologie/research/90630053838221/